This Recommendation mainly describes the maintenance band and requirements for maintenance
test light filtering (such as cut-off bandwidth, isolation and other optical characteristics) for testing
in-service fibre lines without interfering with optical communication signals in access networks.
L.66 provides guidance on the use of an out-of-band remote test system. An alternative approach is
to monitor key parameters of the transmission equipment, such as the OLT transmitted power and
the ONU received power, but this approach is not examined in this Recommendation.